2,406 research outputs found
Normalization of IZF with Replacement
ZF is a well investigated impredicative constructive version of
Zermelo-Fraenkel set theory. Using set terms, we axiomatize IZF with
Replacement, which we call \izfr, along with its intensional counterpart
\iizfr. We define a typed lambda calculus \li corresponding to proofs in
\iizfr according to the Curry-Howard isomorphism principle. Using realizability
for \iizfr, we show weak normalization of \li. We use normalization to prove
the disjunction, numerical existence and term existence properties. An inner
extensional model is used to show these properties, along with the set
existence property, for full, extensional \izfr
An interpretation of the Sigma-2 fragment of classical Analysis in System T
We show that it is possible to define a realizability interpretation for the
-fragment of classical Analysis using G\"odel's System T only. This
supplements a previous result of Schwichtenberg regarding bar recursion at
types 0 and 1 by showing how to avoid using bar recursion altogether. Our
result is proved via a conservative extension of System T with an operator for
composable continuations from the theory of programming languages due to Danvy
and Filinski. The fragment of Analysis is therefore essentially constructive,
even in presence of the full Axiom of Choice schema: Weak Church's Rule holds
of it in spite of the fact that it is strong enough to refute the formal
arithmetical version of Church's Thesis
A Normalizing Intuitionistic Set Theory with Inaccessible Sets
We propose a set theory strong enough to interpret powerful type theories
underlying proof assistants such as LEGO and also possibly Coq, which at the
same time enables program extraction from its constructive proofs. For this
purpose, we axiomatize an impredicative constructive version of
Zermelo-Fraenkel set theory IZF with Replacement and -many
inaccessibles, which we call \izfio. Our axiomatization utilizes set terms, an
inductive definition of inaccessible sets and the mutually recursive nature of
equality and membership relations. It allows us to define a weakly-normalizing
typed lambda calculus corresponding to proofs in \izfio according to the
Curry-Howard isomorphism principle. We use realizability to prove the
normalization theorem, which provides a basis for program extraction
capability.Comment: To be published in Logical Methods in Computer Scienc
Type-Based Termination, Inflationary Fixed-Points, and Mixed Inductive-Coinductive Types
Type systems certify program properties in a compositional way. From a bigger
program one can abstract out a part and certify the properties of the resulting
abstract program by just using the type of the part that was abstracted away.
Termination and productivity are non-trivial yet desired program properties,
and several type systems have been put forward that guarantee termination,
compositionally. These type systems are intimately connected to the definition
of least and greatest fixed-points by ordinal iteration. While most type
systems use conventional iteration, we consider inflationary iteration in this
article. We demonstrate how this leads to a more principled type system, with
recursion based on well-founded induction. The type system has a prototypical
implementation, MiniAgda, and we show in particular how it certifies
productivity of corecursive and mixed recursive-corecursive functions.Comment: In Proceedings FICS 2012, arXiv:1202.317
An Intuitionistic Formula Hierarchy Based on High-School Identities
We revisit the notion of intuitionistic equivalence and formal proof
representations by adopting the view of formulas as exponential polynomials.
After observing that most of the invertible proof rules of intuitionistic
(minimal) propositional sequent calculi are formula (i.e. sequent) isomorphisms
corresponding to the high-school identities, we show that one can obtain a more
compact variant of a proof system, consisting of non-invertible proof rules
only, and where the invertible proof rules have been replaced by a formula
normalisation procedure.
Moreover, for certain proof systems such as the G4ip sequent calculus of
Vorob'ev, Hudelmaier, and Dyckhoff, it is even possible to see all of the
non-invertible proof rules as strict inequalities between exponential
polynomials; a careful combinatorial treatment is given in order to establish
this fact.
Finally, we extend the exponential polynomial analogy to the first-order
quantifiers, showing that it gives rise to an intuitionistic hierarchy of
formulas, resembling the classical arithmetical hierarchy, and the first one
that classifies formulas while preserving isomorphism
Inductive and Coinductive Components of Corecursive Functions in Coq
In Constructive Type Theory, recursive and corecursive definitions are
subject to syntactic restrictions which guarantee termination for recursive
functions and productivity for corecursive functions. However, many terminating
and productive functions do not pass the syntactic tests. Bove proposed in her
thesis an elegant reformulation of the method of accessibility predicates that
widens the range of terminative recursive functions formalisable in
Constructive Type Theory. In this paper, we pursue the same goal for productive
corecursive functions. Notably, our method of formalisation of coinductive
definitions of productive functions in Coq requires not only the use of ad-hoc
predicates, but also a systematic algorithm that separates the inductive and
coinductive parts of functions.Comment: Dans Coalgebraic Methods in Computer Science (2008
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